1. Field of the Invention
The present invention relates to a method for canceling echoes in the voice mail system, and more particularly a method for transmitting a training signal for finding out the line characteristic at the beginning of establishing a speech path.
2. Description of the Related Art
Echo cancelers for communication systems, such as telephone communication systems, are well known in the art. Most common echo cancelers utilize a digital adaptive filter wherein coefficients of the filter are updated according to line characteristics in a communication path. Examples of such echo cancelers, and incorporated herein by reference, are found in: U.S. Pat. No. 5,381,474 to Lahdemaki entitled Method Of Converging An Echo Canceler wherein a test signal is transmitted between ringback (ringing tones) signals to the echo path in the direction of the caller in response to a received ringing voltage from the caller; U.S. Pat. No. 5,400,394 to Vijay R. Raman entitled Adaptive Echo Canceler For Voice Messaging System which uses low-level white noise as a training signal and the white noise burst, masked by masking tones, is transmitted to the caller wherein ring-tones are used as the masking tones; and U.S. Pat. No. 5,553,137 to Scott 0. Nyhart entitled Method And Apparatus For Echo Canceling In A Communication System also uses low-level white noise wherein the white noise is generated in a pseudo-random sequence onto a phone line during inter digit dialing time.
Generally, a voice messaging system such as that disclosed in U.S. Pat. No. 5,400,394 is also known as a voice mail system (VMS). The VMS is a centralized digital facility to record speech messages from telephone callers, and the speech messages are stored in the receiver's individual voice mail box connected with the usual switchboard and accessed/replayed remotely by the called subscriber by means of a special code.
It is well known that when a guidance message is transmitted in the VMS, it is echoed back due to a line impedance mismatch. The reflected guidance message drops, to a very low level, the level of the dual tone modulated frequency (DTMF) signal received by the VMS corresponding to the key pressed by the caller. Consequently, it is impossible for the conventional method to provide a service of superior quality because the VMS cannot detect the low level DTMF signal correctly.
In order to solve the above problem, I have disclosed an echo cancellation technique in Korean Patent Application No. 93-28403 entitled Echo Canceling Circuit Based On The Detection Of Dual Tone Multiple Frequency And Echo Canceling Method, filed on Dec. 18, 1993, in which an echo canceler having an adaptive filter implemented by a digital signal processor (DSP) is installed at the fore-end of the DTMF, thereby adjusting the adaptive filter coefficient value by means of a least mean square (LMS) algorithm so that the differential signal, i.e., error between input data passed through the adaptive filter and through the virtual echo path, is converged to a minimum level in the training mode every time when establishing speech paths. However, it is impossible for the technique of the above Korean Patent Application No. 93-28403 to perform echo cancellation perfectly due to the noises inherent in the trunk line and the subscriber line.
In order to cancel echoes due to such trunk and subscriber line noises, I have disclosed another echo cancellation technique in the Korean Patent Application No. 95-16187 entitled Device And Method For Detecting Dual Tone Multiple Frequency Signals filed on Jun. 17, 1995.
Referring to FIG. 1, the configuration of the voice mail system (VMS) is the same as that of FIG. 4 in the above Korean Patent Application No. 95-16187 which is intended to enable the accurate detection of the DTMF signal despite of the presence of echoes caused by the trunk line noise and/or the subscriber noise.
According to the echo cancellation algorithm disclosed in the above Korean Patent Application No. 95-16187, after finding the basic line characteristic by using a signal close to the training signal for a given period of time (about 150 ms) at the beginning of establishing a speech path, the adaptive filter coefficient is determined on the basis of the resulting line characteristic. However, it is noted that the above training signal used for a specified time interval provides an annoying noise to the caller. Consequently, my voice mail system has the drawback that the DTMF signal received from the caller cannot be detected correctly, and thus the superior quality of the various voice information services can not be guaranteed.